At present, there has been the development and deployment of optical communications systems to provide households with telephone services, Internet services, and cable television services. These systems are replacing the characteristic copper wire cable networks that have been used in the past.
In these optical communications systems, optical signals, whether analog or digital, are distributed from a central office over an optical distribution network (“ODN”) to end users. At the endpoints of this network are optical network terminals (“ONTs”) or optical network units (“ONUs”) that convert the optical signals into electrical signals.
If the system is a fiber-to-the-premises (“FTTP”) system, the ONT would be on the private property of the end user. However, in most other FTTx system configurations, the ONT is on public property. In all of these system configurations, the converted signals usually travel electrically between the ONT and end user devices.
The fiber-to-the-home (“FTTH” or more generally “FTTx”) market is based on a passive optical network (“PON”) that is used for the bi-directional transmission of data between optical line terminations (“OLTs”) located at the central office and ONTs or ONUs located in or at the outside the homes or premises of end users. An example of a typical system that incorporates a PON, OLT, and ONUs is shown in FIG. 1, generally at 100.
Referring to FIG. 1, OLT 102 will serve as a system element that is located at the central office. The OLT terminates the line signals received from end users. OLT 102 is in bi-directional communication with PON 104. The PON includes a network that brings optical fiber cabling and signals all or most of the way to the end user. Depending on where the PON terminates, the network may be described as part of a fiber-to-the-curb (“FTTC”), fiber-to-the-building (“FTTB”), or FTTH system.
PON 104 bi-directionally connects to ONU #1 106, ONU #2 110, and ONU #3 114. Each of ONUs are shown connected to a single end user residence. More specifically, ONU #1 106 is connected to end user residence 108, ONU #2 110 is connected to end user residence 112, and ONU #3 114 is connected to end user residence 116. One of the main purposes of ONUs is to multiplex and demultiplex signals to and from a fiber transmission line from PON 104. Each ONU terminates an optical fiber line and converts the signal to a format suitable for distribution to end user equipment. When used for residential use, a single ONU can serve 1 to 500 dwellings.
The transmission standards for FTTx are based on the International Telecommunications Union (“ITU”) Gigabit PON (“GPON”) or Broadband PON (“BPON”) specifications, and the IEEE Gigabit Ethernet PON (“GEPON”) or Ethernet PON (“EPON”) specifications. These standards are adopted on a regional basis and define the data rates as well as the interoperability with existing networks.
Currently, the gigabit standards for the gigabit Ethernet passive optical network (“GEPON”) in Japan and the gigabit passive optical network (“GPON”) in the United States are the leading the demand for 1.25-2.5 Gbps burst-mode transceivers for FTTx. The target sensitivity at an OLT or ONU transceiver is specified for each standard. For example, the GPON minimum sensitivity for each Class is specified as follows:
Class A−21 dBmClass B−21 dBmClass B+−27 dBmClass C−28 dBm
The sensitivity of a receiver defines the maximum number of users and distances over which an ONU/OLT can operate and how economically a network can be implemented. Therefore, the more sensitive the receiver, the greater the number of users at a given distance.
As is seen above, Class C GPON system sensitivity is greater than Class A GPON sensitivity. Accordingly, Class C GPON system will handle a larger number of users per transmission line unit length.
The graph at FIG. 2, generally at 200, shows an example of the relationship between distance and the maximum number of ONTs (or ONUs) for alternative Classes A, B, and C. At 202, the number of ONTs that may be handled at a 7 km distance is shown. At 208, for Class A systems, it shows that a maximum of 16 ONTs may be handled; at 210, for Class B systems, it shows that a maximum of 40 ONTs may be handled; and, at 212, for Class C systems, a maximum of 101 ONTs may be handled.
At 204, the number of ONTs that may be handled at a 10 km distance is shown. At 214, for Class A systems, it shows that a maximum of 13 ONTs may be handled; at 216, for Class B systems, it shows that a maximum of 32 ONTs may be handled; and, at 218, for Class C systems, a maximum of 81 ONTs may be handled.
At 206, the number of ONTs that may be handled at a 20 km distance is shown. At 220, for Class A systems, it shows that a maximum of 6 ONTs may be handled; at 222, for Class B systems, it shows that a maximum of 15 ONTs may be handled; and, at 224, for Class C systems, a maximum of 39 ONTs may be handled.
There needs to be an improved circuit configuration for a receiver optical sub-assembly (“ROSA”), that will improve the quality of transmissions over both short and long distances so that sensitivity targets specified under the current standards are met or exceeded for a greater number of users to be connected to the system.